Adrenoreceptors, or adrenergic receptors, are sites on effector organs that are innervated by postganglionic adrenergic fibers of the sympathetic nervous system and are classified as alpha-adrenergic and beta-adrenergic receptors. Alpha-adrenergic receptors respond to norepinephrine and to such blocking agents as phenoxybenzamine and phentolamine, whereas beta-adrenergic receptors respond to epinephrine and to such blocking agents as propranolol.
Beta-adrenergic receptors are subclassified as beta-1, beta-2 and beta-3 adrenoreceptors. Beta-1 stimulation causes cardiostimulation, whereas beta-2 stimulation causes bronchodilation and vasodilation.
Beta-3 receptors are found on the cell surface of both white and brown adipocytes where their stimulation promotes both lipolysis and energy expenditure. Agonists selective for beta-3 adrenoreceptors are known to be useful in the treatment of hyperglycemia (diabetes) and obesity in mammals, as well as in the treatment of gastrointestinal disorders and neurogenetic inflammation (U.S. Pat. No. 5,561,142). Additionally, they are known to lower triglyceride and cholesterol levels and to raise high density lipoprotein levels in mammals (U.S. Pat. No. 5,451,677). Accordingly, they are useful in the treatment of conditions such as hyper-triglyceridaemia, hypercholesterolaemia and in lowering high density lipoprotein levels as well as in the treatment of atherosclerotic and cardiovascular diseases and related conditions.
Treatment of such chronic diseases with agonists that are selective for beta-3 adrenoreceptors decreases the potential for undesirable side effects caused by beta-1 or beta-2 receptor stimulation such as increased heart rate (beta-1) and muscle tremor (beta-2). It has now been found that certain novel chroman derivatives are effective as selective beta-3 agonists and useful in the treatment of beta-3 mediated conditions.